NEW YORK — The launch of a tamper-resistant formulation of a popular opioid painkiller in the United States, but not in Canada, may have influenced a spike in dispensing rate of the original at pharmacies in Canadian border cities, according to a study.

Researchers in Canada studies prescriptions dispensed from pharmacies at Niagara Falls, Windsor and Sarnia, Ontario, the cities with the highest volume of U.S.-Canada border crossings in the province. Analyzing data on outpatient prescriptions for Purdue Pharma's OxyContin (oxycodone) extended-release tablets from April 2010 to February 2012, they calculated and compared monthly prescription rates, adjusted per 1,000 people. The study appeared online Tuesday in the Canadian journal Open Medicine.

The researchers found that the number of tablets dispensed near four border crossings in the three cities remained stable, but the rate of dispensing at pharmacies near the Detroit-Windsor Tunnel increased four-fold between August 2010 and February 2011, from 505 tablets per 1,000 people to 1,969, a number that declined to 1,683 in April 2011 following warnings to prescribers and pharmacies and then increasing again in November of that year. Overall, between August 2010 and October 2011, 242,075 "excess" tablets were dispensed near the Detroit-Windsor Tunnel.

According to published reports, the researchers were concerned about the effects that the launch of generic versions of OxyContin could have on health and safety as the drug, which remains a popular target for drug abusers and criminals, becomes more widespread.